Cubic boron nitride is hardest second to diamond and excellent also in thermal stability and chemical stability. Since cubic boron nitride is more stable toward an iron-based material than diamond, a cubic boron nitride sintered material has been used for a tool for machining an iron-based material.
The cubic boron nitride sintered material, however, contains a binder in an amount approximately not lower than 10 volume % and not higher than 40 volume % (10 to 40 volume %) and the binder has been a cause for lowering in strength, heat resistance, and thermal diffusibility of the sintered material. Therefore, in particular in cutting of an iron-based material at a high speed, thermal load has been great, chipping or generation of a crack in a cutting edge has been likely, and lifetime of the tool has been short.
A technique for solving this problem includes a method of manufacturing a cubic boron nitride sintered material with a catalyst instead of a binder. In this method, hexagonal boron nitride is employed as a source material and reaction and sintering thereof is performed by employing magnesium boronitride (Mg3BN3) or the like as a catalyst. Since the cubic boron nitride sintered material obtained with this method is free from a binder, cubic boron nitride is firmly bonded to cubic boron nitride and the cubic boron nitride sintered material is high in thermal conductivity. Therefore, the cubic boron nitride sintered material is used for a heat sink material or a tape automated bonding (TAB) bonding tool. A small amount of the catalyst, however, remains in this sintered material. Therefore, when heat is applied, a small crack tends to be generated due to a difference in thermal expansion between the catalyst and cubic boron nitride and the cubic boron nitride sintered material is not suited for a cutting tool or the like. In addition, the cubic boron nitride sintered material has a large grain size around 10 μm. Therefore, though the cubic boron nitride sintered material is high in thermal conductivity, it is low in strength and cannot withstand cutting applications in which load is high.
A cubic boron nitride sintered material is obtained also by directly converting and simultaneously sintering atmospheric pressure type boron nitride such as hexagonal boron nitride into cubic boron nitride at an ultra-high pressure and a high temperature without using a catalyst (direct conversion sintering). For example, Japanese Patent Laying-Open No. 47-034099 (PTD 1) and Japanese Patent Laying-Open No. 03-159964 (PTD 2) show a method of obtaining a cubic boron nitride sintered material by converting hexagonal boron nitride into cubic boron nitride at an ultra-high pressure and a high temperature. A method of obtaining a cubic boron nitride sintered material by employing a pyrolytic boron nitride as a source material is available. A method of such a type is shown, for example, in Japanese Patent Laying-Open No. 54-033510 (PTD 3) and Japanese Patent Laying-Open No. 08-047801 (PTD 4). In this method, a condition not lower than 7 GPa and 2100° C. is required.
Japanese Patent Publication No. 49-027518 (PTD 5) and Japanese Patent Laying-Open No. 11-246271 (PTD 6) describe a method of obtaining a cubic boron nitride sintered material under a condition milder than the condition above.